1. Field of the Invention
The present invention relates to a developing method and a device therefor for developing an electrostatic latent image with one-component magnetic developer.
2. Description of the Prior Art
There are already known various developing methods utilizing one-component developer. Among these particularly noteworthy is so-called toner transition development, in which one-component developer applied as a thin uniform layer on developer bearing means is brought into facing relationship to an electrostatic latent image bearing surface with a small clearance therebetween whereby the developer is caused to fly from said developer bearing means to the image bearing surface by the electrostatic attractive force to perform image development, as disclosed in the Japanese Patent Publication No. Sho 41-9475 and in the U.S. Pat. No. 2,839,400. Such developing method assures satisfactory development completely without background fog as the developer is not attracted nor brought into contact with the non-image area having no image potential. Besides, the absence of carrier particles eliminates the troubles arising from the change in mixing ratio thereof in the developer or from the time-dependent deterioration of the carrier particles.
Furthermore, the present applicant proposed novel developing methods as disclosed in the U.S. Patent Applications Ser. Nos. 938,101 and 938,494, and Ser. Nos. 58,434 and 58,435.
The former method is featured in positioning one-component magnetic developer, a developer bearing non-magnetic means and a magnetic field generating means in this order to form a thin uniform layer of said developer on said developer bearing means under the effect of the magnetic field generated by said magnetic-field generating means, and maintaining said thin developer layer in facing relation to an electrostatic latent image bearing surface with a small clearance therebetween thereby extending the developer corresponding to the image area by the electrostatic attractive force of said latent image to perform the image development. This method also is capable of providing a completely fog-free visible image as the development is carried out without contact of the developer with the non-image area.
The latter method is featured in positioning one-component magnetic developer, a developer bearing non-magnetic means and a magnetic field generating means in this order to form a thin uniform layer of said developer on said developer bearing means under the effect of the magnetic field generated by said magnetic-field generating means, and maintaining said thin developer layer in facing relation to an electrostatic latent image bearing surface with a small clearance therebetween, wherein the image development is effected by applying an AC developing bias voltage across said clearance between the image bearing surface and developer bearing means and changing said clearance in time. In this method the developer reaches also the non-image area in the initial stage to effect the development of halftone image but reaches only the image area in the later stage, thereby providing the image without background fog and with improved halftone reproducibility in comparison with the above-explained method.
As explained in the foregoing, the developing methods in which a thin layer of one-component developer is faced to the latent image bearing surface are far superior to other conventionally known methods in terms of the developing performance, image reproducibility, service life of the developer etc.
These developing methods, different from the brush development in which brush-shaped developer is maintained in sweeping contact with the latent image bearing surface to obtain a visible image, utilize a thin layer of developer which is spaced from the latent image bearing surface in the absence of an externally applied electric field. FIG. 1 shows an example of developing device employed in such developing method, wherein shown are a latent image bearing member 1, a sleeve roller 2 internally provided with fixed magnetic poles 2a-2d, a fixed magnetic doctor blade 3, a one-component magnetic developer 4, a container 5 therefor, a scraper 6 for removing the developer from the surface of said sleeve roller, and an AC bias voltage source 7 as described in the aforementioned U.S. Patent Applications Ser. Nos. 58,434 and 58,435. In such device the magnetic pole 2b positioned inside the sleeve 2 and said magnetic blade 3 provide a strong magnetic field for obtaining a practical thin developer layer of 50-100.mu. in thickness, and the insulating one-component developer is subjected to frictional charging through contact with the rotating sleeve 2 and is defined in thickness by the magnetic field present between the sleeve 2 and the blade 3 before being supplied to the developing area D. The clearance between the sleeve 2 and the blade 3 is an important factor in forming a strong magnetic field and has to be as narrow as 200 to 400.mu., for example for forming a flux density of 800 gauss on the sleeve, corresponding to the above-mentioned layer thickness. For this reason the magnetic blade 2 has to be finished with a high precision at the pointed end thereof. Also the magnetic thickness defining in said narrow clearance causes a strong distortive force in the developer 4 leading to developer caking and thus giving rise to a streak in the developer layer. Such a streak is also caused by foreign matter stuck in said clearance. Also certain developers cannot satisfactorily fill the already consumed portions of the developer layer, thus leading to the so-called sleeve ghost phenomenon. Furthermore, incidentally aggregated developer, if deposited on the sleeve 2, creates a relatively thick developer layer around such aggregate, which can hardly be restored to the thin state even after many turns of the sleeve. Also the amount of charge on the developer, which is generally determined by the frictional charging between the sleeve 2 and the insulating developer, becomes reduced after prolonged use as the sleeve surface is covered by minute particles of resin employed in the developer, eventually leading to so-called sleeve deterioration involving insufficient developer application on the sleeve. For these reasons there is often needed scraping means, as the scraper 6 in FIG. 1, for removing the developer from the sleeve 2 before developer application.